Project Summary/Abstract Peer relationships are highly influential in adolescent social development1,2 and may be protective against anxiety3 and depression.4,5 Investigating markers of social engagement may elucidate targets for understanding processes underlying individual socio-emotional trajectories. Two major gaps exist in the literature on adolescent social engagement: 1) The behavioral and attentional mechanisms underlying successful social engagement are not well understood. 2) Studies examining potential mechanisms a) rely on computer-based tasks that emulate social interactions (e.g., cyberball,6 chatroom task,7,8 virtual school9) or b) forgo measures of neural processing or eye-tracking in favor of more naturalistic social interactions. In the current study, 13-year-olds will be paired with a novel peer and provided the opportunity to engage in a naturalistic social interaction. During the social interaction, behavioral and mobile eye-tracking (MET) data will be collected. Importantly, the mobile eye-tracking system will also capture each participant?s perspective of the social interaction. Gaze measures will be used to examine social attention during a naturalistic interaction. The perspective videos will be used to examine mentalizing system and threat circuit connectivity in response to the naturalistic interaction. Specifically, participants will ?re-live? the interaction in the scanner, watching both positive and negative moments from both their, and their partner?s, perspectives. The design will enable the identification of multiple markers of naturalistic social engagement and the research team will address three main aims: 1) Identify individual differences in adolescent social attention and behavior during a naturalistic peer interaction. 2) Examine individual differences in mentalizing system and threat circuit connectivity during a re-experienced peer interaction. 3) Examine the potential moderating effect of social attention on individual differences in mentalizing system and threat circuit connectivity patterns. Findings will provide valuable information about markers of social engagement, paving the way for future work examining how these markers may develop and interact to support the emergence of psychopathology, particularly anxiety. The current study is designed with an integrated training plan that will prepare the fellowship applicant for a future career as a developmental neuroscientist. The three overarching training goals are: 1) Obtain training in how to measure individual differences in adolescent peer interactions. 2) Obtain training in neuroscience methodology and multilevel modeling. 3) Advance training and professional development in preparation for an independent research career. These training goals will effectively prepare the applicant for the next steps in the desired career path of becoming a developmental neuroscientist as well as lay the foundation for a future research program examining how interacting processes, behavioral, attentional and neural, underlie the development of social engagement, which also may modulate risk for psychopathology.